The wars started between countries due to territorial disputes, nationalism, political tensions and economic competition.
These things can escalate into conflict when the diplomatic solutions fail and a perceived imbalance of power or threat emerges.
And in most cases a specific event such as an assassination or invasion, can also act as a catalyst which triggers a wider war.
Countries often go to war with each other to gain control over land for resources, living space or even a strategic advantage.
Historical examples also include border conflicts and disputes over access to valuable resources such as minerals or water.
And economic rivalries, which include competition for markets, trade routes or resources which can lead to conflict.
And the desire to dominate global trade or secure access to vital resources can also drive nations into war.
Other reasons for countries to go to war are from nationalism, political tensions and other specific events.
Intense nationalistic fervor, in which a country believes itself to be superior to others can also fuel aggression and this can lead to a desire to expand territory or exert dominance over other nations.
And ideological differences, political instability, or the rise of authoritarian regimes can also create an environment that is conducive to conflict and alliances between nations can also create a system in which a conflict involving one nation can quickly escalate into a wider war that involves many.